Polyimide matrix electrolyte and improved batteries therefrom

ABSTRACT

A battery includes an anode, a cathode, and a polymer matrix electrolyte (PME) separator disposed between the anode and the cathode. The PME separator includes a polyimide, at least one lithium salt in a concentration of at least 0.5 moles of lithium per mole of imide ring provided by the polyimide, and at least one solvent intermixed. The PME is generally homogeneous as evidenced by its high level of optically clarity. The battery can be a lithium ion or lithium metal battery.

CROSS REFERENCE TO RELATED APPLICATION

[0001] Not applicable.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

[0002] Not applicable.

FIELD OF THE INVENTION

[0003] The invention generally relates to polymer matrix electrolytesfor use as an electrolyte or in primary and rechargeable lithium metaland lithium ion batteries.

BACKGROUND

[0004] The demand for new and improved electronic devices, such ascellular phones, notebook computers and compact camcorders, has demandedenergy storage devices having increasingly higher specific energydensities. For example, the telecommunication industry is activelyseeking alternate energy storage devices for its outside plant back-uppower sources for telecommunication stations to replace the currentstandard valve-regulated lead acid batteries. Moreover, the automotiveindustry is in need of high specific energy density batteries for thedeveloping electric and hybrid vehicles market. A number of advancedbattery technologies have recently been developed to service thesedevices and markets, such as metal hydride (e.g., Ni-MH), nickel-cadmium(Ni—Cd), lithium batteries with liquid electrolytes and recently,lithium batteries with polymer electrolytes.

[0005] Lithium batteries have been introduced into the market because oftheir high energy densities. Lithium is atomic number three on theperiodic table of elements, having the lightest atomic weight andhighest energy density of any solid material. As a result, lithium is apreferred material for batteries, having very high energy density.Lithium batteries are also desirable because they have a high unit cellvoltage of up to approximately 4.2 V, as compared to approximately 1.5 Vfor both Ni—Cd and Ni-MH cells.

[0006] Lithium batteries can be either lithium ion batteries or lithiummetal batteries. Lithium ion batteries intercalate lithium ions in ahost material, such as graphite, to form the anode. On the other hand,lithium metal batteries use metallic lithium or lithium alloys for theanode.

[0007] The electrolyte used in lithium batteries can be a liquid or apolymer based electrolyte. Lithium batteries including liquidelectrolytes have been on the market for several years. Lithium ionrechargeable batteries having liquid electrolytes are currently massproduced for applications such as notebook computers, camcorders andcellular telephones. However, lithium batteries having liquidelectrolyte technology have several major drawbacks. These drawbacksrelate to cost and safety and stem from use of a liquid electrolyte. Theliquid electrolyte generally requires packaging in rigid hermeticallysealed metal “cans” which can reduce energy density. In addition, forsafety reasons, lithium ion rechargeable batteries and lithium-metalprimary batteries having liquid electrolytes are designed to ventautomatically when certain abuse conditions exist, such as a substantialincrease in internal pressure which can be caused by internal orexternal overheating. If the cell is not vented under extreme pressure,it can explode because the liquid electrolyte used in liquid Li cells isextremely flammable.

[0008] Lithium batteries having solid polymer electrolytes represent anevolving alternative to lithium batteries having liquid electrolytes.Solid polymer electrodes are generally gel type electrolytes which trapsolvent and salt in pores of the polymer to provide a medium for ionicconduction. Typical polymer electrolytes comprise polyethylene oxide(PEO), polyether based polymers and other polymers which are configuredas gels, such as polyacrylonitrile (PAN), polymethylmethacrylate (PMMA)and polyvinylidine fluoride (PVDF). The polymer electrolyte generallyfunctions as a separator, being interposed between the cathode and anodefilms of the battery.

[0009] Because its electrolyte is generally a non-volatile materialwhich does not generally under normal operating conditions leak, alithium battery having a polymer electrolyte is intrinsically safer thana lithium battery having a liquid electrolyte. Moreover, polymerelectrolytes eliminate the need for venting and package pressure controlwhich are generally required for operation of lithium batteries havingliquid electrolytes. Thus, polymer electrolytes make it possible to usea soft outer case such as a metal plastic laminate bag, resulting inimprovement in weight and thickness, when compared to liquid electrolytecan-type Li batteries.

[0010] Each cathode, separator and anode combination forms a unitbattery cell. Practical lithium batteries, such as those having polymerelectrolytes, are generally prepared by stacking a number of batterycells in series and/or parallel to achieve desired battery capacity.

[0011] Lithium metal polymer (LMP) rechargeable batteries offer improvedperformance as compared to Li ion batteries, particularly highercapacity. LMP batteries result from the lamination/assembly of threetypes of main thin films: a film of positive electrode comprising amixture of a polymer and an electrochemically active material such aslithium vanadium oxide, an electrolyte film separator made of a polymerand a lithium salt, and a negative electrode film comprising metalliclithium or a lithium alloy. A known limitation of LMP batteries,especially for automotive applications is that the polymers presentlyused provide their peak ionic conductivity at elevated temperatures.These LMP batteries must therefore be heated to produce peak poweroutput.

[0012] A problem with Li metal and lithium alloy batteries is thatlithium, in its metallic form, is highly reactive. As such, it presentsunique difficulties in rechargeable configurations. Repeatedcharge/discharge cycles can cause a build-up of surface irregularitieson the lithium metal electrode. External pressure is generally necessaryfor prolonged performance of a lithium metal battery.

[0013] These irregular structures, known as dendrites, can grow to suchan extent that they penetrate the separator between positive andnegative electrodes and create an internal short circuit. At best, thisphenomenon shortens the useful life of a rechargeable Li-metal batteryto generally about 900 cycles or less.

[0014] Many performance parameters of lithium batteries are associatedwith the electrolyte choice, and the interaction of the selectedelectrolyte with the cathode and anode materials used. High electrolyteionic conductivity generally results in improved battery performance.The ionic conductivity of gel polymer electrolytes have been reported tobe as high as approximately 10⁻⁴ S/cm at 25° C. However, it is desirablefor the ionic conductivity of the polymer electrolyte to reach evenhigher values for some battery applications. In addition, it would alsobe desirable to enhance the electrochemical stability of the polymerelectrolyte towards anode and cathode materials to improve batteryreliability, as well as storage and cycling characteristics.

[0015] While gel polymer electrolytes represent an improvement overliquid electrolytes in terms of safety and manufacturability, safetyissues remain because gel polymers trap solvent on its pores and underextreme conditions (e.g. heat and/or pressure) can still escape andcause injury. In addition, gel polymer electrolytes cannot generallyoperate over a broad temperature range because the gel generally freezesat low temperatures and reacts with other battery components or melts atelevated temperatures. Moreover, electrode instability and resultingpoor cycling characteristics, particularly for metallic lithiumcontaining anodes, limits possible applications for such batteriesformed with gel polymer electrolytes.

[0016] Alternative polymer materials have been actively investigated toprovide improved characteristics over available polymer choices. Forexample, U.S. Pat. No. 5,888,672 to Gustafson et al. ('672 patent)discloses a polyimide electrolyte and a battery formed from the samewhich operates at room temperature and over a broad range oftemperatures. The polyimides disclosed are soluble in several solventsand are substantially amorphous. When mixed with a lithium salt, theresulting polyimide based electrolytes provide surprisingly high ionicconductivity. The electrolytes disclosed in '672 are all opticallyopaque which evidences some phase separation of the various componentscomprising the electrolyte. Although the electrolytes disclosed by the'672 patent can be used to form a polymer electrolyte and a batterytherefrom which provides an improved operating temperature range, easeof manufacture, and improved safety over batteries formed fromconventional gel polymer electrolytes, it would be helpful if theelectrolyte stability over temperature and pressure as well as the ionicconductivity could be improved.

SUMMARY OF THE INVENTION

[0017] A battery includes an anode, a cathode, and a polymer matrixelectrolyte (PME) separator disposed between the anode and the cathode.The PME separator includes a polyimide, at least one lithium salt in aconcentration of at least 0.5 moles of lithium per mole of imide ringprovided by the polyimide and at least one solvent, all intermixed. ThePME is generally homogeneous as evidenced by its high level of opticalclarity. As used herein, when the PME is referred to as beingsubstantially optically clear. The phrase “substantially opticallyclear” regarding the PME refers to the PME being at least 90% clear(transmissive), preferably at least 95%, and most preferably being atleast 99% clear as measured by a standard turbidity measurement,transmitting through a normalized 1 mil film using 540 nm light.

[0018] The battery can be a lithium ion, lithium metal battery or alithium metal alloy battery. A repeat unit weight per imide ring of thepolyimide can be no more than 350, no more than 300, or no more than250. The polyimide is preferably soluble at 25° C. in at least onesolvent selected from the group consisting of N-methylpyrrolidinone(NMP), dimethylacetamide (DMAc) and dimethylformamide (DMF).

[0019] The ionic conductivity of the polymer electrolyte at 25° C. is atleast 1×10⁻⁴ S/cm, and preferable at least 3×10⁻⁴ S/cm. The Li salt canbe selected from a variety of salts including LiCl, LiBr, Lil, LiClO₄,LiBF₄, LiAsF₆, LiPF₆, LiCF₃SO₃, LiBOB, LiN(CF₃SO₂)₂, and lithiumbis(trifluorosulfonyl)imide (LiTFSI).

[0020] The cathode can include an ion conducting polymeric binderintermixed with an intercalation material. In this embodiment, thepolymeric binder can include at least one polyimide, such as thepolyimide used in the PME.

[0021] The cathode can include an electrochemically active materialselected from the group consisting of LiFePO₄, Li_(x)Ni_(y)CO_(z)O₂,LiV_(x)O_(y), Li_(x)Mn_(y)O_(z), LiV_(x)O_(y), Li_(x)Mn_(y)O_(z),LiCoO₂, LiNiO₂ and LiTiS₂,

[0022] The PME permits the battery to be highly stable over temperatureand pressure extremes. The battery can demonstrate no significant changein open circuit voltage (OCV) and capacity following heating at 125° C.for at least 5 minutes while under a pressure of at least 200 psi, ormore preferably no significant change in OCV and capacity followingheating at 140° C. for at least 10 minutes while under a pressure of atleast 250 psi.

[0023] The battery can be a bicell. In this embodiment, the cathodehaving a PME coating thereon is preferably folded over to sandwich theanode.

[0024] The salt complexes with the polyimide. Evidence for complexing isshown by the salt and polyimide not providing any absorption peaksbetween 1630 and 1690 cm⁻¹, while the PME formed from the mixtureprovides at least one absorption between about 1630 and 1690 cm⁻¹.

[0025] The battery can be a packaged battery, wherein the packagecomprises packaging material surrounding the battery, the packagingmaterial being laminated to all exterior surfaces of the battery. In apreferred embodiment, a frame is included in the packaging process, theframe having an opening to accommodate the battery therein, where thebattery is disposed within the opening and the packaging material isdisposed thereon. In the frame embodiment, the packaged battery canprovide a thickness uniformity throughout within ±1 mil.

[0026] A method for forming a battery includes the steps of providing acathode layer disposed on a cathode current collector, overcoating amixture of polyimide, lithium salt and solvent on the cathode layer,drying the mixture to remove at least a portion of the solvent, whereinan electrolyte separator bonded to the cathode (electrolyte/cathode) isformed. An anode layer is then disposed on the electrolyte/cathode. Theelectrolyte separator is preferably a polymer matrix electrolyte (PME),wherein the polyimide, lithium salt and solvent are intermixed, thelithium salt in a concentration of at least 0.5 moles of lithium permole of imide ring provided by the polyimide, the PME beingsubstantially optically clear. The anode is preferably a lithium metalcomprising anode, although it can be a Li ion anode.

[0027] The battery can be formed by laminating only two layers, a firstlayer being the PME disposed on the cathode and a second layer being theanode. The PME coated cathode is then laminated with the anode to formthe cell. In one embodiment of the invention, the method can include thestep of folding a PME coated cathode over the anode, wherein a bicell isformed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] A fuller understanding of the present invention and the featuresand benefits thereof will be accomplished upon review of the followingdetailed description together with the accompanying drawings, in which:

[0029]FIG. 1(a)-(m) illustrates the repeat unit structure for severalsuitable polyimides, according to an embodiment of the invention.

[0030]FIG. 2 is a table illustrating ionic conductivity and theresulting film composition at 20.5° C. for a PME comprising thepolyimide shown in FIG. 1(m), LiTFSi salt (2.2×) and the solvent gammabutyrolactone (GBL), according to an embodiment of the invention.

[0031]FIG. 3(a)-(d) are plots of the ionic conductivity of a PMEcomprising the polyimide shown in FIG. 1(m) and the lithium salt LiTFSi(2.2×) as a function of solvent load and temperature for the solventsTMS, PC, GBL, and NMP, respectively.

[0032]FIG. 4 is a plot of ionic conductivity of a PME comprising thepolyimide shown in FIG. 1(m) and a lithium salt (LiTFSi; 2.2×) as afunction solvent/polyimide ratio at 20 C, where the solvent is GBL.

[0033]FIG. 5(a) is FTIR data for an exemplary polyimide without theaddition of a Li salt or any solvent.

[0034]FIG. 5(b) is FTIR data for an exemplary Li salt.

[0035]FIG. 5(c) is FTIR data for the exemplary polyimide whose FTIR datais shown in of FIG. 5(a) intermixed with and the exemplary Li salt whoseFTIR data is shown in FIG. 5(b) evidencing the emergence of a doubletabsorption peak at between about 1630 cm⁻¹ and 1690 cm⁻¹.

[0036]FIG. 6 is a table which includes dianhydrides and aromaticdiamines that can be used for making polyimides presented herein.

[0037]FIG. 7 shows an assembly process for forming an electrochemicalbicell which is ready for packaging, according to an embodiment of theinvention.

[0038]FIG. 8 shows an assembly process for packaging the electrochemicalbicell produced by the assembly process shown in FIG. 7.

[0039]FIG. 9(a) is a SEM evidencing a defect free PME.

[0040]FIG. 9(b) is a SEM showing a smooth and uniform interface betweena PME disposed on a lithium vanadium oxide (LVO) comprising cathodelayer.

[0041]FIG. 10 shows the charge/discharge profile of a typical cellaccording to the invention, the cell including a lithium vanadium oxidecathode, a Li metal anode and a PME separator, according to anembodiment of the invention.

DETAILED DESCRIPTION

[0042] The invention describes a substantially optically clearelectrolyte separator matrix which includes a polyimide, a lithium saltand some solvent. The lithium salt is in a concentration of at least 0.5moles of lithium per mole of imide ring provided by the polyimide. Thesolvent is generally a low molecular weight, low viscosity liquid whichswells the polyimide at low concentration and at a sufficiently highconcentration allows the polyimide, salt and the solvent to becomehomogeneously mixed. As used herein, the substantially optically clearelectrolyte separator matrix is referred to as a “polymer matrixelectrolyte” or PME. The PME can be used for batteries, supercapacitorsand other applications. The high level of optical clarity provided bythe PME evidences the homogeneity of the PME comprising its respectivecomponents (polymer, salt and the solvent) as any significant phaseseparation would reduce optical clarity.

[0043] The PME can be used to form both lithium ion and lithium metalcontaining batteries. Unlike gel polymer electrolytes, once the PME isformed, there is generally no free solvent or identifiable pores. Forexample, using a SEM providing a 50 A resolution, no pores in the PMEcan be identified. Instead, the solvent is integrated with the polymerand the lithium salt in a homogeneous and substantially optically clearmatrix. In addition, unlike conventional gel polymers where the polymeronly provides mechanical support, the polymer together with the salt andthe solvent participate in ionic conduction.

[0044] The PME provides high current carrying capacity, cyclingstability and maintains this performance level across a wide temperaturerange, such as at least from −40° C. to 100° C. For example, the PME canwithstand high temperatures and pressures with small changes in opencircuit voltage and capacity, such as the conditions generally used inthe hot lamination processes used in credit card manufacturing, or eventhe more rigorous conditions associate with a typical injection moldingprocess. For example, hot lamination processes used in credit cardassembly generally utilize a temperature of about 115-150° C. for 5 to15 minutes under a pressure of about 200-250 psi. The performance ofbatteries formed using the PME experience no significant change in opencircuit voltage (OCV) or capacity from the temperature and pressureconditions provided by a typical credit card lamination process.

[0045] As used herein, the phrase “no significant change in OCV andcapacity” following a lamination or molding process refers to a batteryincluding the PME which demonstrates an OCV and capacity which shiftsless than 15%, preferably less than 10%, and more preferably less than a5% following the lamination or molding process.

[0046] For example, a lithium metal battery having a lithium vanadiumoxide comprising cathode was found to provide a shift in OCV of 1.4% anda commensurately small reduction in capacity following a laminationprocess comprising a temperature of 140 C for 15 minutes under apressure of 220 psi. Moreover, batteries formed using the PME are quiteflexible and can exceed ISO standards ISO/IEC 10373 and 14443.

[0047] Thus, PME based batteries are particularly well suited for useinside credit cards, smart labels, and other small devices which requirehigh temperature/high pressure lamination processing and can benefitfrom an on board power supply. Regarding credit card and relatedapplications for the invention, the high level of battery stability toboth mechanical and thermal stresses provided by the PME permitsbatteries according to the invention to withstand harsh credit cardprocessing conditions.

[0048] The PME is generally based on one or more polyimides, whichunlike other polymer-based electrolytes, participate in ionicconductivity through the presence of imide rings and other polar groups.Other polymer types which include highly polar groups having functionalgroups that can complex with lithium salts and participate in ionicconduction include polybenzimidazoles and polyamide-imides. Accordingly,these polymer types may also include species useful in forming a PME.

[0049] Polyimides are reaction products of a condensation reactionbetween diamines and dianhydrides to initially form a poly (amic-acid)intermediate. Either or both the diamine and dianhydride reagent can bemixture of diamine or dianhydride species. The poly amic-acidintermediate is then converted to a fully imidized polyimide.

[0050] The properties of the resulting polyimide formed depend on theselection of the particular diamines and dianhydride monomers.Polyimides are generally known to provide high thermal stability,chemical resistance and high glass transition temperatures, such as 200°C. to more than 400° C.

[0051] The current invention includes the identification of polyimidesdistinct from those disclosed in the '672 patent and has found theaddition of solvent to some of these distinct polyimides can result inthe formation of a PME. Some of the polyimides identified herein providerepresent newly synthesized polymers. Unlike the polymers disclosed in'672, the polymers disclosed herein form a substantially homogeneousmatrix material (PME) when combined with an appropriate concentration ofsalt to achieve a practical ionic conductivity level and the solvent. Asnoted above, the homogeneity of the PME is evidenced by the high levelof optical clarity provided which is shown in Example 1. In contrast,the electrolytes disclosed in '672 are non-homogeneous mixture, asevidenced by their opaqueness also shown in Example 1 which isindicative of phase separation of the respective components.

[0052] The Inventors have identified and synthesized improved polyimidesfor use in forming PMEs by qualitatively relating certain polymerparameters to ionic conductivity. Imide ring density is believed toexplain why polyimide films, when loaded with lithium salt, showsignificant ionic conductivity, even in the absence of solvent. Repeatunit weight per imide ring is one measure of imide ring density and iscalculated by dividing the molecular weight of the entire repeat unit ofthe respective polyimides by the number of imide rings within therespective repeat units.

[0053] Imide rings may provide the equivalent of a high dielectricconstant to materials because of the high electron density provided bythe rings. Accordingly, it is believed that the interaction between theimide rings and the lithium ion is a factor in determining the ionicconductivity of a PME. Thus, improved polyimides for use as PMEs can begenerally selected for further consideration by first calculating thenumber of imide rings (and to a lesser degree other highly polar groups,such as sulfone, carbonyl and cyanide) per molecular repeat unit in agiven polyimide. The more imide ring function present per unit weight,the higher the average dielectric strength equivalence of the polymer.Higher equivalent dielectric strength is believed to generally lead toimproved salt interaction, which can improve the ionic conductivity ofthe PME.

[0054] Alternatively, a quantity roughly inverse to imide ring densityreferred to as repeat unit weight per imide ring can be calculated toalso compare the relative concentration of imide rings in polyimides. Asthe repeat weight per imide ring decreases, imide rings become anincreasingly greater contributor to the repeat unit as a whole. As aresult, as the repeat weight per imide ring decreases, the equivalentdielectric constant and ionic conductivity of the polyimide generallyincrease.

[0055] Polyimides which provide high helium permeability may generallyproduce higher ionic conductivity and thus form better PMEs. At 25° C.,the measured He permeability of most polyimides according to theinvention has been found to be at least 20 barrers. The He permeabilitycan be at least 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75 or 80barrers, or more.

[0056] Helium permeability may be measured in the following suggestedmanner. The gas permeability (GP) of a gas component through a givenfilm having a thickness (I) and area (A) can be determined by applying apressure (p) across the film which is equal to the upstream pressureminus the downstream pressure of the gas component and measuring theresulting steady state rate of gas flow (f) permeating through the filmat STP.

GP=(fl)/(Ap)

[0057] Preferred units for GP is (cm²)(sec)⁻¹(cmHg), where 1 barrer isdefined as the GP multiplied by 10¹⁰.

[0058] It is believed that the maximum ionic conductivity of the PMEgenerally occurs when the polyimide/salt/solvent combination creates acompletely homogenous, clear matrix. Any phase separation is expected toreduce the ionic conductivity values because phase separation wouldincrease the tortuosity within the PME.

[0059] The maximum electrolyte conductivity of the PME at a giventemperature generally occurs when the polyimide/salt/solvent matrix hasa ratio within a specified range. The optimum salt concentration isgenerally in the range of 0.5 to 2.0 moles Li per mole of imide ring fora polyimide. Too little salt generally does not provide a sufficientnumber of ions to contribute to the charge transfer. Too much saltgenerally leads to phase instability and possible precipitation of thepolyimide and resulting loss of homogeneity which can be verified by aloss in optical clarity.

[0060] The solvent is believed to play the role of a swelling agent forthe polymer to move the backbone chains slightly apart and thereforeenables greater ionic diffusion coefficients. The solvent also acts as asolvation carrier for the ions. Typically the solvent is chosen to alsobe stable within the limitations of the type of battery chosen. Forexample, for a lithium ion type battery the solvent must be stable up tothe reduction potential of lithium metal. The amount of solvent can beoptimized depending upon the either the conductivity desired or thesoftening point of the matrix at some required temperature. Solvents canbe selected from a group of solvents including gamma butyrolactone(GBL), propylene carbonate, N-methylpyrrolidinone (NMP),tetrahydrothiophen-1,1-dioxide (TMS), polycarbonate (PC) and dimethylformamide (DM F).

[0061]FIG. 1(a)-(m) illustrates the repeat unit structure for severalpolyimides, according to an embodiment of the invention. The repeat unitfor a polymer referred to as Polyimide A is shown in FIG. 1(a). Thispolyimide can be formed by reacting PMDA [89-32-7] with TMMDA[4037-98-7]. This high molecular weight polymer is soluble in NMP for alimited number of hours thus a lithium salt containing electrolyte couldnot be produced.

[0062] The repeat unit for the polymer referred herein to as Polyimide Bis shown in FIG. 1(b). This polyimide can be formed by reacting 90 mole% PMDA [89-32-7], 10 mole % 6FDA [1107-00-2] dianhydrides and thediamine TMMDA [4037-98-7]. The polymer formed was soluble indefinitelyin NMP at 20% by weight.

[0063] The repeat unit for the polymer referred herein to as Polyimide Cis shown in FIG. 1(c). This polyimide can be formed by reacting 85.7mole % PMDA [89-32-7], 14.3 mole % 6FDA [1107-00-2] and TMMDA[4037-98-7]. The polymer formed was soluble indefinitely in NMP at 20%by weight.

[0064] The repeat unit for the polymer referred herein to as Polyimide Dis shown in FIG. 1(d). This polyimide can be formed by reacting 80 mole% PMDA [89-32-7], 20 mole % PSDA [2540-99-0] and TMMDA [4037-98-7]. Thepolymer formed was soluble indefinitely in NMP at 20% by weight.

[0065] The repeat unit for the polymer referred herein to as Polyimide Eis shown in FIG. 1(e). This polyimide can be formed by reacting BPDA[2421-28-5] and 3,6 diaminodurene [3102-87-2]. The polymer formed wassoluble indefinitely in NMP at 20% by weight.

[0066] The repeat unit for the polymer referred herein to as Polyimide Fis shown in FIG. 1(f). This polyimide can be formed by reacting 6FDA[1107-00-2] and 3,6-diaminodurene [3102-87-2]. The polymer formed wassoluble indefinitely in NMP at 20% by weight and is also soluble inacetone, GBL, DMAc, and DMF.

[0067] The repeat unit for the polymer referred herein to as Polyimide Gis shown in FIG. 1(g). This polyimide can be formed by reacting PSDA[2540-99-0] and TMMDA [4037-98-7]. The polymer formed was solubleindefinitely in NMP at 20% by weight and is soluble in GBL.

[0068] The repeat unit for the polymer referred herein to as Polyimide His shown in FIG. 1(h). This polyimide can be formed by reacting 6FDA[1107-00-2] and TMMDA [4037-98-7]. The polymer formed was solubleindefinitely in NMP at 20% by weight and is soluble in acetone, GBL,DMAc, and DMF.

[0069] The repeat unit for the polymer referred herein to as Polyimide Iis shown in FIG. 1(i). This polyimide can be formed by reacting BPDA[2421-28-5] and 4,4′-(9-fluorenylidene)dianiline [15499-84-0]. Thepolymer formed was soluble indefinitely in NMP at 20% by weight.

[0070] The repeat unit for the polymer referred herein to as Polyimide Jis shown in FIG. 1(j). This polyimide can be formed by reacting PMDA[89-32-7], 33.3 mole % TMPDA [22657-64-3] and 66.7 mole % TMMDA[4037-98-7]. The polymer formed was soluble indefinitely in NMP at 20%by weight.

[0071] The repeat unit for the polymer referred herein to as Polyimide Kis shown in FIG. 1(k). This polyimide can be formed by reacting PMDA[89-32-7] and DAMs [3102-70-3]. The polymer formed was soluble only fora short time in NMP.

[0072] The repeat unit for the polymer referred to herein as Polyimide Lis shown in FIG. 1(l). This polyimide can be formed by reacting PMDA[89-32-7] and 4-isopropyl-m-phenylenediamine [14235-45-1]. The polymerformed was soluble indefinitely in NMP at 20% by weight.

[0073] The repeat unit for the polymer referred herein to as Polyimide Mis shown in FIG. 1(m). This polyimide can be formed by reacting PMDA[89-32-7], 33.3 mole % DAMs [3102-70-2] and 66.7 mole % TMMDA[4037-98-7]. The polymer formed was soluble indefinitely in NMP at 20%by weight.

[0074] The PME includes at least one lithium salt. High PME ionconductivities are generally achieved using a high salt content, such asfrom about 0.5 to 2.0 moles of Li per mole of imide ring for polyimidepolymers. However, the concentration of lithium salt can be 0.6, 0.7,0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1,2.2, 2.3, 2.3, 2.4 or 2.5 moles Li per mole of imide ring of thepolyimide.

[0075] Salt concentrations are also described herein as a ratio of theweight of the salt to weight of the polymer (e.g. polyimide) excludingthe salt. For example, a 1× concentration corresponds to an equal amountof salt and polymer, while a 2×salt concentration corresponds to twicethe salt concentration as compared to the polymer concentration. Thelithium salt can generally be any lithium salt known in the art.Preferably, lithium salts are chosen from LiCl, LiBr, Lil, Li(ClO₄),Li(BF₄), Li(PF₆), Li(AsF₆), Li(CH₃CO₂), Li(CF₃ SO₃), Li(CF₃ SO₂)₂N,Li(CF₃ SO₂)₃, Li(CF₃ CO₂), Li(B(C₆H₅)₄, Li(SCN), and Li(NO₃), lithiumbis(trifluorosulfonyl)imide (LiTFSI), LiBOB, and LiSCN. Preferably, thesalt is either Li(PF₆) or LiTFSi.

[0076] Polyimide structures other than the exemplary structures shown inFIG. 1 can be good candidates for use in forming a PME matrix.Characteristics that a good PI should display, such as solubility in NMP(or other suitable solvent) to at least 20 wt % and a low repeat unitweight per imide ring can be a first criteria. However, the solubilityof the PI itself or the solution stability when combined with a highconcentration of a lithium salt applicable for battery applications isnot easily predictable.

[0077] Behavior in solution can not generally be ascertained by simplylooking at the polymer repeat unit structure. However, it can bedetermined from looking at the structure whether the polymer willpossibly be crystalline, or whether it would have a high or low relativedensity in the solid phase. The polyimides are preferablynon-crystalline, low-density repeat structures because they would bemost likely to be very soluble. However, changing a single side groupfrom a methyl to an ethyl can cause the properties of the PI to bechanged from useful to of no use. A suggested meaningful screeningprocedure for alternative polyimides for battery applications is to takea polyimide that is known to have a good solubility in the solventitself and then try to introduce the lithium salt at 0.5 mole Li perimide ring or more and see if (i) the resulting PI/salt/solvent solutionis clear and homogenous for a few hours and (ii) the film cast from thatsolution remains substantially clear when most of the solvent (e.g. 95%)has been evaporated.

[0078]FIG. 2 is a table illustrating the ionic conductivity at 20.5° C.and the corresponding film composition for a polyimide based PMEcomprising the polyimide shown in FIG. 1(m), LiTFSi salt (2.2×) and thesolvent GBL, according to an embodiment of the invention. Roomtemperature ionic conductivity values for PME films were calculated frommeasured impedance using a complex impedance analyzer. Measurementsusing the impedance analyzer were made in the frequency range of 1 MHzto 0.1 Hz. A value for the resistance of the film, R, was taken as theintercept with the real axis of a Nyquist plot, or by an extrapolationof the high frequency portion of the response curve to the real axis.Measurements were also made on an LCR meter at a fixed frequency of 10kHz where the resistance values were read directly. The ionicconductivity was calculated using the formula:

C(Seimens/centimeter)=t/R×10⁻⁴

[0079] where t is film thickness in microns and R is the measured filmresistance in Ohms.

[0080] As shown in FIG. 2, for the PMEs tested, the maximum ionicconductivity at 20.5° C. was about 1×10⁻⁴ S/cm for a 1× saltconcentration and 4×10⁻⁴ S/cm for a 2.2× concentration, where the PMEfilm comprised 37.9% GBL. The lower table portion shown in FIG. 2 showsthe minimum solvent level required to provide a conductivity of 1×10⁻⁴S/cm for 1.0, 1.4, 1.8 and 2.2×salt concentrations.

[0081] The range of solvent in FIG. 2 is expressed as the solvent/PIratio and runs from about 1 to about 2. The lower value is where onearbitrarily defines a conductivity as being high enough for a givenapplication, such as 1.0E-4 S/cm. The maximum conductivity was found tooccur between a solvent/PI ratio of 1.5 to 2.0 depending upon the saltconcentration.

[0082] The upper solvent/PI limit near 2 is controlled by the mechanicalproperties of the PME. At a high solvent/PI ratio the PME film becomesvery soft and may not be usable over an extended temperature range as aseparator for a battery. Applied to lithium metal batteries, As apractical matter, the solvent is chosen to be moderately stable againstlithium metal, high boiling (>150 C), capable of dissolving the chosensalt and swelling the PI, and finally it should have a low viscositywhich promotes a high conductivity for the salt/solvent combination byitself, as well as for the PME.

[0083] To optimize the respective PI, salt and solvent concentrations, asuggested procedure is described below. in a first step, a PI species isselected which is soluble in an appropriate solvent, such as NMP or GBL.All the polyimides shown in FIG. 1 except the polyimide shown in FIG.1(k), are soluble in NMP. In the second step, a desired lithium salt isselected and an initial concentration is selected, such as a mass ratiothat gives one lithium ion per imide ring provided by the selected PI.The solvent concentration can then be varied from a solvent/PI ratio ofabout 0.8 to 2.0 while measuring the ionic conductivity for severalsolvent/PI data points in between. This can be repeated at saltconcentrations both higher and lower. There are generally severalcombinations of PI/salt/solvent by mass that will give similar ionicconductivities, such as within a factor of two.

[0084]FIG. 3(a)-(d) are plots of the ionic conductivity of a PMEcomprising the polyimide shown in FIG. 1(m) and the lithium salt LiTFSi(2.2×) as a function of solvent load and temperature for the solventsTMS, PC, GBL, and NMP, respectively. These curves generally do not showa clear conductivity maximum because the salt content was 2.2×, and as aresult, the amount of solvent needed to provide the maximum ionicconductivity was large and generally impractical.

[0085]FIG. 4 is a plot of ionic conductivity of a PME comprising thepolyimide shown in FIG. 1(m) the lithium salt (LiTFSi; 2.2×) as afunction solvent/polyimide ratio at 20° C., where the solvent is GBL.The ionic conductivity is seen to increase for increasing solvent topolyimide ratio up to about a GBL/polyimide ratio of about 1.5, wherethe ionic conductivity levels off at a value of about 7×10⁻⁴ S/cm.

[0086] The ionic conductivity of PMEs based on polyimides is believed tobe related to the degree the polyimides associate with alkali salts,particularly Li salts. This association is likely related to theformation of complexes between polar groups (e.g. imide and benzene) ofthe polyimide and the salt. Evidence for this association can be foundby measuring the absorption frequencies displayed by the polymer whenintermixed with the salt and comparing this data to the absorptionfrequencies displayed by the polyimide and the salt by themselves.

[0087] For a polyimide, characteristic absorption frequencies arebelieved to be related to the imide rings and benzene rings thatcomprise the polyimide backbone. These absorption peaks are at about1778 cm⁻¹ and 1721 cm⁻¹ for the imide ring and 730 cm⁻¹ for the benzenering. Common Li salts, including LiPF₆, LiBOB, Lil and LiTFSi do notshow absorption at these frequencies, nor any absorption peaks betweenabout 1630 and 1690 cm⁻¹. However, when certain polyimides whichassociate strongly with Li salts are combined with those salts, theresulting polyimide/salt film show the emergence of a very strongdoublet with a first peak at about 1672 cm⁻¹ and a second peak at about1640 cm⁻¹.

[0088] The first absorption frequency is present when a significantconcentration of salt is in the polyimide film, such as at least 33 wt.%, but does not change significantly with increasing salt concentration.Thus, this peak can be used to indicate the interaction, provided asufficient salt concentration is present. The magnitude of the secondpeak is nearly proportional to the ratio of the Li ion concentration tothe imide rings and can be used to assess the extent of the interactionbetween the polyimide and the salt.

[0089] Polymers other than polyimides which provide a high relativelocal electron density (e.g. comparable to electron density adjacent toimide rings in polyimides) and the potential for charge separation dueto multiple bond conjugation may form a polymer/salt complex and displaycharacteristic absorption peaks based on the complex formed, providedthey are soluble. Thus, absorption spectroscopy, such as FTIR can beused to identify these soluble non-polyimide polymers which showevidence of complex formation for use as a PME.

[0090]FIG. 5(a) is FTIR data for an exemplary polyimide, the polyimideshown in FIG. 1(c), without the addition of a Li salt or any solvent.Absorption peaks related to the imide rings are shown at about 1778 cm⁻¹and 1721 cm⁻¹ and at 730 cm⁻¹ for the benzene ring. This polyimide doesnot show any detectable absorption peaks between about 1630 and 1690cm⁻¹.

[0091] Common Li salts, including LiPF₆, LiBOB, Lil and LiTFSi do notshow absorption between 1630 and 1690 cm¹. FIG. 5(b) shows an FTIR forLiBOB confirming the absence of infrared absorption between about 1630and 1690 cm⁻¹.

[0092]FIG. 5(c) shows the FTIR the polyimide shown in FIG. 1(c) combinedwith LiBOB. The resulting polyimide electrolyte shows a very strongdoublet with a first peak at about 1672 cm⁻¹ and a second peak at 1640cm⁻¹.

[0093] Thus, FTIR measurements taken from exemplary PMEs according tothe invention provide evidence that the lithium salt forms a complexwith the imide rings of the polyimide. This is likely one of the mainreasons why the polyimide/lithium salt combination, in a dry film, hasno apparent structure or crystallinity. The lack of crystallinity can beshown by a flat DCS trace. This evidence has also been found of aninteraction between the lithium salt and the polyimide as the PME filmis optically clear whether as a free standing film or as an overcoatedfilm.

[0094] The invention can be used in a wide variety of applications. Bothprimary and rechargeable batteries formed from either lithium ion orlithium metal anodes can be formed using the invention. In a preferredembodiment, a lithium metal battery having a PME is formed. In thisembodiment, the anode can be formed from lithium metal or a lithiumalloy. Lithium alloys can include lithium-aluminum,lithium-aluminum-silicon, lithium-aluminum-cadmium,lithium-aluminum-bismuth, lithium-aluminum-tin or lithium-titaniumoxide. The lithium content in these lithium alloys may be up to 99.98wt. %. The overall lithium content in the anode is based on the requiredcapacity of the battery.

[0095] The cathode material for the lithium metal battery can comprisean ionic conductive polymer binder, such as the polyimides describedherein, an electronically conductive material (e.g. graphite), and anelectrochemically active material. The electrochemically active materialis preferably selected from MnO₂, the various lithium vanadium oxides(Li_(x)V_(y)O_(z)), lithium transition metal oxides, such asLi_(x)Mn_(y)O_(z) (e.g. LiMn₂O₄), LiCoO₂, LiNiO₂, Li₄Ti₅O₁₂,LiV_(x)O_(y), and other materials such as metal sulfides (e.g. TiS₂) andLiFePO₄. Similarly, cathodes and anodes in the case of Li ion batteriespreferably include polyimides as binder matrices, such as thosedisclosed herein. For the Li ion or lithium metal battery, the anode,cathode and PME separator can all include the same or differentpolyimides.

[0096] Polyimides are generally prepared by reacting one or morediamines and one or more dianhydride monomers in a step-wisecondensation reaction. Preferred diamines and dianhydrides are shown inFIG. 6.

[0097] The purity of the monomeric reactants has been found to beimportant. Aromatic diamines are known to be air sensitive. Thus, apurification process generally includes removing a portion of theoxidized diamine material. Following purification, aromatic diaminestypically become white. However, absolutely pure diamines are difficultto achieve, and usually unnecessary. Recrystallization of the diaminefrom a mixture of alcohol and water has been found to be both effectiveand sufficient. The collected crystals are preferably dried under avacuum because the water used for crystallization may not be easilyremovable at room temperature.

[0098] Dianhydrides are also preferably purified and can be purifiedusing at least two methods. A preferred purification method isrecrystallization from acetic anhydride. Acetic anhydride closesreactive anhydride rings. However, removal of acetic acid produced issometimes difficult. Washing with ether or MTBE can be used to removethe residual acetic acid. The dianhydride crystals generally requiredrying in a vacuum oven. An alternate method is to quickly wash thecrystals with dry ethanol to remove open anhydride rings and then torapidly dry the washed powder in a vacuum oven. This method works forpurifying BPDA if the incoming purity is at least approximately 96%.

[0099] The condensation reaction is performed to the poly (amic-acid)stage. One mole of diamine is reacted with about 1.005 to 1.01equivalents of dianhydride. Excess dianhydride is generally preferablyused to ensure that the dianhydride reagent determines the end group ofthe reaction product, the dianhydride excess ensuring that polymerchains are terminated with anhydride end groups which are known tomaximize stability of the polymer product. In addition, such an excesscompensates for traces of water which can react with anhydride groups.

[0100] The solution is preferably about 15 to 20 weight % total in NMPsolvent. The diamine is usually dissolved first, preferably undernitrogen, although this is not required for all diamines. Thedianhydride is then added. A large scale reaction preferably adds thedianhydride in portions because there is some heat of reaction and it isdesirable to keep the temperature below 40° C. For example, a smallscale reaction of 50 grams could add the dianhydride all at once,provided there is good initial mixing.

[0101] The dianhydride generally dissolves much more slowly than thediamine. For a small-scale reaction, the best and simplest way toperform the reaction is to use a cylindrical jar with apolytetrafluoroethylene sealed cap. This jar can be shaken by hand inthe beginning and then put on a rolling mill at slow speed to turn thejar smoothly. The reaction proceeds best at room temperature over a twoto twelve hour period. Once the initial reaction is complete, asevidenced by a substantially constant solution color and viscosity, thesolution is ready for imide ring closure.

[0102] The chemical reaction to form the polyimide from the poly(amic-acid) can be performed with 1.1 equivalents of acetic anhydrideper imide ring and 1 equivalent of pyridine as a ring closure catalyst.Pyridine can be added and rolling preferably continued until mixed. Theacetic anhydride can cause a portion of the amic-acid polymer toprecipitate.

[0103] The polymer should be redissolved before heating. One needs tocarefully heat the jar to more than 80° C. but less than 90° C. It isbest to use an oven and occasionally remove the jar to shake. Heating isperformed for 60 minutes at full heat or until the color changes arecomplete. The jar is put on a rolling mill to cool while turning. Oncethe solution returns to room temperature, a fully imidized polymerdissolved in NMP results. However, the solution generally also has someresidual pyridine, acetic acid, and may include some acetic anhydride.The solution is expected to be stable for reasonable periods of time.

[0104] A preferred method for collecting the product from a small-scalereaction is to precipitate the polymer into methyl alcohol. This removesthe solvent load and traces of monomers that did not react. It also canbe used, in conjunction with the observed viscosity, to estimate thequality of the polymerization. A good, high molecular weight polyimidewill precipitate cleanly into methyl alcohol with little or nocloudiness. The viscosity should be high (e.g. 8000 to 10,000 cp).

[0105] The precipitated polymer is preferably washed two or more timesuntil the smell of pyridine is slight. The polymer can then be air driedfor a few hours in a hood and then dried in a vacuum oven at 125° C.overnight. The volume of methyl alcohol is preferably about a gallon per100 grams of polymer.

[0106] On a larger scale, water can be used to remove solvents or tracematerials. However, water is expected to be a less efficient than methylalcohol for this purpose. A final soak with methyl alcohol is preferablyadded before drying if water is used with large quantities of polymer.

[0107] In another aspect of the invention, a two piece battery assemblyis disclosed. The two piece assembly comprises overcoating an electrodewith a PME to form an electrode/separator and subsequent assembly (e.g.lamination) with the other electrode.

[0108] In a conventional “gel” electrolyte technology a cathode slurryof electrochemically active material, conductive carbon and binder ismixed in a bulk solvent with the solvent functioning as a plasticizerfor the binder material. The slurry is coated upon a current collectorsubstrate and the bulk solvent removed. The anode is typicallycarbonaceous and made from a slurry of graphite, binder and plasticizingsolvent. The bulk solvent is removed after coating on the currentcollector. The separator is coated as a free standing film from a slurryof non-ionically conductive polymer binder, inorganic support fillermaterial and plasticizing solvent. The cathode, separator and anode areplaced together and laminated under heat and pressure. The remainingplasticizer is then removed via an extraction process typicallyutilizing a hot solvent. The cell is removed and a liquid electrolyte isintroduced into the system to fill the “pores” created via removal ofthe plasticizer. The cell is then packaged in a foil packaging material.Thus, conventional assembly processes comprise the joining of threeseparate components.

[0109] In contrast, in the two piece battery assembly a cathode slurryof active material, conductive carbon, polyimide binder and lithium saltis mixed in a bulk solvent. The slurry is coated upon a metal currentcollector substrate and the solvent is removed. Subsequently, the coatedcathode is overcoated with a PME comprising a mixture of polyimide,lithium salt and bulk solvent, and then dried to remove solvent with aneffective amount of solvent retained for conductivity purposes, such as5 to 50 weight % versus polyimide plus salt. At this point theovercoated cathode has become both the cathode and the PME separator. Ananode layer is then placed over the PME coated cathode, thus providing abattery assembly using only two components.

[0110]FIG. 7 shows a preferred assembly “folding” process related to thetwo component battery assembly process described above. A PME coatedcathode on a cathode current collector is first provided as describedabove. Prior to placing the anode on the PME separator/cathode, in step710 the surface of the PME overcoated cathode is sprayed with a smallamount of solvent for adhesion purposes. In step 720 a Li anode, such asa Li metal strip having an area slightly less than the area of the PMEcoated cathode is then placed on the PME coated cathode. Alternatively,for a graphitic anode, the anode could also be coated, dried, thenovercoated on the PME coated cathode. An anode tab, such as a nickeltab, is then placed on the anode in step 730. A cell fold is thenperformed in step 740 by wrapping the PME coated cathode over thelithium metal anode (or graphitic anode) as shown in FIG. 7. In step 750a bicell battery 755 having an anode tab which is ready for packaging isproduced. Bicells provide twice the capacity of conventional cells whilehaving the same footprint of the conventional cell.

[0111]FIG. 8 shows an assembly process for packaging the electrochemicalbicell 755 produced by the assembly process shown in FIG. 7, where thesteps of the assembly process shown in FIG. 7 are repeated on the top ofFIG. 8 for convenient reference. Although the process shown in FIG. 8 isapplied to a bicell battery, the described process is in no way limitedto bicells. For example, standard cells according to the invention canbe packaged as shown in FIG. 8.

[0112] In step 810 packaging frame material is rolled out from asuitable roller, the frame material being a material such as PET, havinga thickness substantially the same as the battery itself to fit aroundthe battery. In step 820 an opening is created in the frame materialwhich is slightly larger than the cell 755 to be inserted using a windowpunch or equivalent. A registration hole is also shown which can becreated in step 820.

[0113] Tab sealant is applied to the package frame material orpre-applied to the tab strip. In step 830 a cathode tab is fed, cut andapplied to the frame material. Lower packaging material having a lowtemperature heat sealant and moisture barrier is then applied to theframe. The interior heat sealant layer is a material that not only heatseals to itself, but also bonds strongly to the current collector of thebattery, and preferably activates at between about 90 C to 100 C. Thecell 755 is then preferably pick and placed into the cavity formed bythe punched out package frame and onto the lower packaging materialattached to the bottom of the frame in step 840. The lower packagingmaterial is preferably heated (e.g. 90 C) prior to placing the cell 755thereon to promote adhesion. Upper packaging material having a lowtemperature heat sealant and moisture barrier, which can be the samecomposition as lower packaging material, is then applied in step 850.Optionally, a binder material can be included on the outside of thelower and upper packaging material for applications such as theinsertion into a credit card.

[0114] Instead of using a perimeter seal used in a conventionalstep-down seal process, the entire surface of the battery along with theperimeter of the packaging material is heat laminated in step 860 toform a finished bicell battery 865 having substantially the samethickness dimension throughout. Although assembly of only a singlebicell is shown, the assembly process shown in FIG. 8 can package one ormore cells in a single package. A lamination temperature such as 110° C.for typically 5 to 15 seconds under a pressure sufficient to produce auniform heat seal but not too high as to distort the battery and/or thepackage is used to provide the lamination to form the finished bicellbattery 865. The substantially constant thickness of bicell 865facilitates integration into a card, such as a credit card, and alsoreduces surface defects.

[0115] As used herein, “substantially the same thickness dimensionthroughout the battery” refers to a thickness over the entire area ofthe packaged battery being ±1 mil. A totally laminated battery withpackage 865 results which behaves as a single laminated layer.

[0116] Although not shown in FIG. 8, a step-down seal process which doesnot utilize a frame can be used. Instead, an elastomeric laminationplate set is used. This allows intimate contact of the packagingmaterial all over the surface of the battery, the packaging materialnecking down to the side seal area where the packaging materiallaminates to itself.

[0117] Thus, several packaging new packaging aspects are describedabove. First, the battery package material is laminated to the entirebattery. Previous lamination work discloses only lamination of a portionof the battery. Second, the use of a frame to surround the cell permitsproduction of highly planar batteries having substantially the samethickness dimension throughout.

EXAMPLES

[0118] The present invention is further illustrated by the followingspecific examples. The examples are provided for illustration only andare not to be construed as limiting the scope or content of theinvention in any way.

Example 1 Optical Clarity Measurements for Commercially AvailablePolyimide/Salt/Solvent as Compared to Exemplary PMEs According to theInvention

[0119] The purpose of this example is to demonstrate the homogenousnature of the PME formed from polyimides that have strong interactionswith a lithium salt through demonstrated optical transmission. PME filmswere generated from polyimides shown in FIGS. 1(b), (c) and (m), while aMatrimid 5218 p/salt/solvent was generated for comparison. The PMEs andMatrimid based film were cast upon a clear polyethylene terephthalatesupport film from NMP and dried for three hours in a vacuum oven at 120°C. The residual solvent content was approximately 4 wt %. All PMEcompositions and the Matrimid formulation were 32% polyimide, 64% LiTFSisalt, and the remainder being the small amount of solvent. Allmeasurements were performed using 540 nm light. Film Thickness %Transmission Polyimide Type (mils) Absorbance for a 1 mil film Matrimid5218 0.70 1.903  0.2% FIG. 1(b) Pl 1.68 0.005 99.3% FIG. 1(c) Pl 0.660.000  100% FIG. 1(m) Pl 0.87 0.000  100%

[0120] As demonstrated from the table above, the Matrimid polyimide doesnot form a clear homogenous composition when the lithium saltconcentration is 64%. Moreover, the Matrimid polyimide does not form aclear homogenous composition even at substantially lower saltconcentrations, such at a concentration of 0.5 moles of lithium salt perimide ring provided by the polyimide. Polyimides including a lithiumsalt in a concentration of 0.5 moles (or less) of lithium salt per imidering provided by the polyimide generally lack sufficient minimum ionicconductivity to be a useful electrolyte separator for an electrochemicalcell.

[0121] Although only data for polyimides shown in FIGS. 1(b), (c) and(m) are provided above, all polyimides shown in FIG. 1 ((a) through (m))have been shown to form homogenous, clear films having opticalproperties as shown above for polyimides (b), (c) and (m). This PMEproperty is desired for maximum film stability and ionic conductivity.

Example 2

[0122] Since the PME is a homogeneous or near homogeneous matrixmaterial, batteries formed using the PME exhibit a unique ability tohandle exposure to high temperatures and pressures, and still maintainits performance characteristics. Cell assembly is typically performed atelevated temperatures (e.g. 140° C.) and pressure (e.g. 125 psi) forabout 10 minutes, so subsequent exposures to similar or less rigorousconditions do not effect cells manufactured according to the invention.Significantly, it has been found that cells according to the inventioncan withstand conditions significantly more rigorous as compared to theabove-referenced cell assembly conditions.

[0123] Typical card manufacturing techniques require hot lamination at125° C. for 5 or more minutes in an hydraulic press exerting over 250psi. Batteries according to the invention have demonstrated fullfunction following this process with no significant change in opencircuit voltage (OCV) or capacity following typical laminationprocessing. In contrast, other battery technologies currently availablewill fail to operate (e.g. short) following conventional laminationprocessing or significantly lose capacity and/or OCV.

[0124] Batteries according to the invention can withstand conditionsmore rigorous than typical lamination processing. For example, a batteryaccording to the invention was subjected to an injection molding processto see if the battery could withstand the conditions encountered duringan injection molding process. Injection molding process wereinvestigated for molding the polymers polyethylene-terephthalate (PET)and polyvinyl chloride (PVC) to encapsulate batteries according to theinvention. The PET molding was performed at 295° C. and PVC at 200° C.,with the mold pressure for both processes being 5000 psi. Both testsshowed batteries according to the invention enduring the respectiveinjection molding processes fully functional, with no significant changein capacity or OCV.

Example 3

[0125] As noted in the background, Li battery systems with liquidelectrolytes or gel polymers having solvent in the pores of the polymerraise safety issues. Although the PME based cells according to theinvention generally include some solvent, the solvent is complexed withthe polymer and salt, not trapped in a porous film (gel electrolyte) orfreely available (liquid electrolyte).

[0126] A rigorous test of a direct short circuit was applied to a 9AmpHour cell formed from a PME. The cell ramped to 45° C., then slowlyreturned to room temperature at full discharge. The cell was able to berecharged and returned to normal stable cycling. No cell mechanicaldefects, outgassing, flames, or bubbling were observed. This is asurprising result which provides additional evidence regarding thestability of batteries formed using the invention.

Example 4

[0127] Scanning electron microscope (SEM) micrographs were taken toevaluate the morphology of the PME according to the invention and theinterface between the PME and a lithium vanadium oxide (LVO)/polyimidecathode film. FIG. 9(a) shows two (2) SEM micrographs of the PME alonewhich each evidence a substantially defect free and a porosity freefilm. The thickness of the PME films shown were both about 75 μm.

[0128]FIG. 9(b) shows a SEM of an interface between a PME film (20 μm)disposed on a lithium vanadium oxide/polyimide cathode layer (55 μm).The lower images are blowups of the interfaces shown at the top of FIG.9(b). The interfaces shown are quite uniform and smooth.

Example 5

[0129]FIG. 10 shows the charge/discharge profile of a typical cellaccording to the invention, the cell including a lithium vanadium oxide(LVO) cathode, a Li metal anode and a PME separator, according to anembodiment of the invention. The cell capacity was 25 mAh. The y-axisrepresents cell voltage, while the x-axis represents total time. Thecell demonstrates good cycling stability during a cycling of over 300hours.

[0130] While the preferred embodiments of the invention have beenillustrated and described, it will be clear that the invention is not solimited. Numerous modifications, changes, variations, substitutions andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as described in theclaims.

We claim:
 1. A battery comprising: an anode; a cathode, and at least onepolymer matrix electrolyte (PME) separator disposed between said anodeand said cathode, said PME separator comprising a polyimide, at leastone lithium salt in a concentration of at least 0.5 moles of lithium permole of imide ring provided by said polyimide, and at least one solventintermixed, said PME being substantially optically clear.
 2. The batteryof claim 1, wherein said anode comprises lithium ion intercalationmaterial.
 3. The battery of claim 1, wherein said anode compriseslithium metal.
 4. The battery of claim 1, wherein said anode comprises alithium metal alloy anode.
 5. The battery of claim 1, wherein a repeatunit weight per imide ring of said polyimide is no more than
 350. 6. Thebattery of claim 1, wherein a repeat unit weight per imide ring of saidpolyimide is no more than
 300. 7. The battery of claim 1, wherein arepeat unit weight per imide ring of said polyimide is no more than 250.8. The battery of claim 1, wherein said polyimide is soluble at 25° C.in at least one solvent selected from the group consisting ofN-methylpyrrolidinone (NMP), dimethylacetamide (DMAc) anddimethylformamide (DMF).
 9. The battery of claim 1, wherein the ionicconductivity of said polymer electrolyte at 25° C. is at least 1×10 4S/cm.
 10. The battery of claim 1, wherein the ionic conductivity of saidpolymer electrolyte at 25° C. is at least 3×10⁻⁴ S/cm.
 11. The batteryof claim 1, wherein said Li salt is at least one selected from the groupconsisting of LiCl, LiBr, Lil, LiClO₄, LiBF₄, LiAsF₆, LiPF₆, LiCF₃SO₃,LiBOB, LiN(CF₃SO₂)₂, and lithium bis(trifluorosulfonyl)imide (LiTFSI).12. The battery of claim 1, wherein said cathode comprises an ionconducting polymeric binder intermixed with an intercalation material.13. The battery of claim 12, wherein said polymeric binder comprises atleast one polyimide.
 14. The battery of claim 1, wherein said cathodefurther comprises an electrochemically active material selected from thegroup consisting of LiFePO₄, Li_(x)Ni_(y)CO_(z)O₂, LiV_(x)O_(y),Li_(x)Mn_(y)O_(z), LiV_(x)O_(y), Li_(x)Mn_(y)O_(z), LiCoO₂, LiNiO₂ andLiTiS₂.
 15. The battery of claim 1, wherein said battery provides nosignificant change in OCV and capacity following heating at 125° C. forat least 5 minutes while under a pressure of at least 200 psi.
 16. Thebattery of claim 1, wherein said battery provides no significant changein OCV and capacity following heating at 140° C. for at least 10 minuteswhile under a pressure of at least 250 psi.
 17. The battery of claim 1,wherein said battery is a bicell, wherein said cathode is folded over tosandwich said anode.
 18. The battery of claim 1, wherein said salt andsaid polyimide do not provide any absorption peaks between 1630 and 1690cm⁻¹, said PME providing at least one absorption between about 1630 and1690 cm⁻¹.
 19. The battery of claim 1, further comprising a packagecomprising packaging material surrounding said battery to form apackaged battery, said packaging material laminated to all exteriorsurfaces of said battery.
 20. The battery of claim 1, further comprisinga package comprising packaging material surrounding said battery and aframe having an opening to accommodate said battery therein to form apackaged battery, wherein said battery is disposed within said opening.21. The battery of claim 20, wherein said packaged battery provides athickness uniformity throughout within ±1 mil.
 22. A method for forminga battery, comprising the steps of: providing a cathode layer disposedon a cathode current collector; overcoating a mixture of polyimide,lithium salt and solvent on said cathode layer; drying said mixture toremove at least a portion of said solvent, wherein an electrolyteseparator bonded to said cathode (electrolyte/cathode) is formed, anddisposing an anode layer on said electrolyte/cathode.
 23. The method ofclaim 22, wherein said electrolyte separator is a polymer matrixelectrolyte (PME), wherein said lithium salt is in a concentration of atleast 0.5 moles of lithium per mole of imide ring provided by saidpolyimide, said polyimide, said lithium salt and said solvent beingintermixed, said PME being substantially optically clear.
 24. The methodof claim 22, wherein said anode is a lithium metal comprising anode. 25.The method of claim 23, wherein said battery is formed by laminatingonly two layers, a first layer being said PME disposed on said cathodeand a second layer being said anode.
 26. The method of claim 25, furthercomprising the step of folding said PME disposed on said cathode oversaid anode, wherein a bicell is formed.